Remembrance of Zion in their past captivity

Psalm 137 refers, and alone does to give the full history of
Israel's sorrows to Babylon, which has only a mystic fulfilment in
the latter days, but has its importance, because at that time was
the closing of the period of the divine presence in Jerusalem, and
the setting up of the power of the Gentiles. But faith could not
content itself in a strange land nor sing the Lord's songs there;
for they were not a heavenly people hence they turn to Jerusalem,
which faith never forgets. Babylon is to be destroyed and her
judgment is desired; Edom's enmity not forgotten. The object of the
psalm is to bring out their attachment to Zion in their captivity;
there was no separation of heart from it in the strange land.